The use of CRTs (cathode ray tubes) for displaying various types of alphanumeric and pictorial information has increased rapidly during recent years along with the expanding role of computers and data processing equipment. CRT display monitors are finding increasing use particularly in the area of computer generated graphics in which pictorial or graphics information generated by a computer is displayed on the CRT screen for analysis. In many of these applications, particularly those involving graphical analysis, definition of the display image on the screen is less than completely satisfactory.
In connection with conventional television type raster scanning, definition is defined as the number of scanning lines which comprise each picture frame of the image to be displayed. Definition is dependent, in part, on the thinness of each scanning line; current conventional CRT display monitors produce scanning lines having a thickness typically between 0.015 and 0.0075 inches, and state of the art monitors may achieve a line thickness of 0.005 inches. Scanning lines of this thickness are well within the range necessary to produce extremely high definition display images. Definition, as defined above however, does not reflect the total amount of information, i.e. the total number of discrete elements or "pixels" which can be utilized, but rather merely deals with a measure of minimum line width, regardless of the number of lines actually displayed. In contrast, the present invention relates to increasing the overall amount of information which can be displayed; the quality of information density will therefore be hereinafter referred to as image "definition".
Prior art CRT display monitors typically utilize a picture frame comprising 262 non-interlaced lines of image data interlaced to form a picture field. Objectionable flicker occurs at refresh rates below approximately 50 Hz in connection with non-interlaced scanning. Flicker may be eliminated through the use of persistance phosphors, but this approach suffers from certain drawbacks such as decreased image brightness and the tendency to "burn". The deflection yokes employed in conventional monitors for moving the electron beam of the tube across the screen are limited in the rate at which the beam may be moved across each line; currently, the maximum scanning rate of prior art, single beam monitors operating at 60 Hz refresh and using a special deflection yoke and drive circuit is approximately 60,000 lines per second.
Although specially designed CRT monitors of the type having a single scanning beam have, in the past, been adapted to exceed the conventional 15,720 scanning lines per second, such monitors are particularly expensive to produce due to the additional high speed logic and control circuitry required by their design, and in any event such specially designed monitors are not compatible with many computer installations which are based on standard television type formats.
In connection with many types of computer generated and computer aided graphics, it is often necessary to simultaneously display a plurality of superimposed images on the screen which are generated from different data sources. In the past, it has been necessary to provide relatively complex mixing circuits for combining the video signals in a manner which allows a single electron beam to form each of the images to be displayed. This approach to the problem has not gained widespread acceptance because it was relatively expensive in terms of the hardware which was required.
Multiple beam CRTs are known per se in the art, as exemplified by U.S. Pat. Nos. 2,978,608, 3,140,473 and 3,671,957. However, none of these prior patents disclose raster scan type monitors capable of generating high definition images. The device shown in U.S. Pat. No. 2,978,608 is not of a true raster scan type and is not capable of producing grey scales. U.S. Pat. No. 3,671,957 discloses a monitor in which each gun paints different areas of the screen, rather than adjacent lines on the screen.
Accordingly, it is a primary object of the present invention to provide a raster scan type CRT display capable of providing high definition display images, but which is relatively simple in design and is readily compatible with existing data processing and television systems.
Another object of the present invention is to provide a monitor of the type described above which employs a plurality of electron scanning beams in which the associated scanning spots are maintained in precise vertical alignment with each other.
A still further object of the invention is to provide a monitor of the type discussed above which includes provision for adjusting the vertical spacing between the scanning lines.
Another object of the invention is to provide a raster scan display monitor having character generation circuitry comprising a plurality of discrete matrix memories which may be simultaneously addressed using the same character designation.
A further object of the invention is to provide a multi-beam scanning display as discussed above in which each of the beams is controlled by different sets of data sources thereby eliminating the need for special circuitry for mixing signals derived from differing data sources.